The present invention relates generally to broadcast systems, and more particularly to systems and methods for synchronizing broadcast transmissions.
A cable modem termination system (“CMTS”) refers to equipment typically found in a cable company's headend. A CMTS can be used to provide high speed data services, such as cable internet or Voice over IP (VoIP), to cable subscribers. In a typical configuration, the CMTS routes data traffic between Ethernet interfaces (such as an IP network) and RF interfaces (e.g., HFC cable). HFC is an acronym for hybrid fiber/coaxial, and an HFC network is a communications network that uses a combination of coaxial cable and fiber optic connections.
The modular CMTS (“M-CMTS”) architecture separates a conventional CMTS into two parts, referred to as the CMTS core (or the M-CMTS core) and the EQAM. The CMTS core provides such functionality as IP networking, MAC timing and framing, and service flow management. The EQAM, which stands for edge quadrature amplitude modulation, includes physical layer (PHY) circuitry that provides such functionality as modulation and frequency up-conversion for transmission of data packets over HFC cables. While the modulators (EQAMs) can be part of the cable provider headend, they may also be at a remote facility.
In a typical M-CMTS implementation, the M-CMTS core communicates with the EQAM over a regional area network (e.g., a packet-based network such as an IP network), and a DOCSIS® timing interface server (DTI server) maintains a consistent timing reference among the data sources, the M-CMTS core, and the EQAM(s). The EQAM(s) synchronize their system clocks to the DTI server and generate modulated symbols to the HFC network.
Although IP networks potentially provide advantages in view of their lower cost, their packet-based nature means that no data clocks are delivered, the data crossing IP networks have varying delay and jitter, and data may be lost. With synchronizing to a reference clock, the EQAMs are able to correct any timing delay and jitter introduced by IP networks.
Although the present invention relates primarily to downstream transmissions, there will be brief references to the upstream communications carried out, for example, by cable modems on the cable network. For example, synchronization such as mentioned above remains essential to cable networks because the physical transmission medium (in this case coaxial cable) is shared by all cable modems on a network. The M-CMTS core synchronizes the EQAM devices to schedule, correct, and insert MPEG timestamps for video. A cable modem receives its synchronization from an EQAM device so that it is synchronized to other cable modems to properly transmit to the upstream receiver. This process ensures that all cable modems sharing the HFC cable and CMTS do not interfere with each other.
As increasing demands on IP-based services from hybrid networks for rich multimedia content, such as high-definition VOD, IPTV, and gaming, the data bandwidth will quickly exceed the M-CMTS core capacity to synchronize the contents to the DTI server. Accordingly, rich multimedia contents may be tunneled directly to the EQAMs (bypassing the M-CMTS core). However, this requires that the EQAMs deliver modulated contents to customers' CPE, but the QAM channels are not synchronized to traditional DOCSIS channels. Furthermore, many data source servers may generate their contents with respect to reference clocks independent of the DTI servers.